William Hague has warned that the Syrian civil war was turning into this
century's "greatest humanitarian catastrophe" as a new report set
out systematic violations of the rules of war.

Hosting a meeting of the G8 foreign ministers in London, Mr Hague declared the outside world must mount a much stronger intervention to curtail the crisis.

Britain invited four leaders of the Syrian Opposition Coalition to be present on the sidelines of the meeting in an effort to increase support for the regime's opponents.

"I will be joining and convening some of those meetings to discuss the urgent humanitarian needs and the urgent need for a political and diplomatic breakthrough," said Mr Hague. "This is turning into the greatest humanitarian catastrophe of the 21st century so far and we cannot watch this happen."

Leaders of the Syrian National Council (SNC) on Wednesday held talks with those foreign ministers willing to meet them, securing a pledge of more support from John Kerry, the US Secretary of State.

However the effort was overshadowed by a declaration from Jabhat al-Nusra, the most effective fighting force in the opposition, of loyalty to al-Qaeda.

An audio recording of the al-Nusra leader, Abu Mohammad al-Golani, formally pledged allegiance to al-Qaeda's Ayman al-Zawahiri, just a day after fundamentalists in Iraq had announced a cross border alliance.

Rebels from Jabhat al-Nusra at Taftanaz air base in the Idlib province in January (AP)

A report from Human Rights Watch said Syrian jets and helicopters had repeatedly attacking civilians by targeting bakeries and hospitals.

"In village after village, we found a civilian population terrified by their country's own air force," said Ole Solvang, one of the researchers responsible for the survey, Death from the Skies.

"These illegal air strikes killed and injured many civilians and sowed a path of destruction, fear, and displacement."

Syrian forces began using helicopters as attack weapons in March of last year, as the uprising turned into a fully-fledged insurgency and rebel fighters began to occupy towns, villages and suburbs across the country.

But the first sustained aerial bombardments began last July after the Free Syrian Army launched a surprise attack seizing half of Aleppo and some Damascus suburbs.

The first recorded jet attack took place on July 24 in Aleppo city.

Since then, the survey says, attacks have happened daily, with an opposition monitoring organisation relying on local activists, the Violations Documentation Centre, recording the names and date and place of death of 4,472 individuals, some rebels but more than 4,000 of them civilians.

HRW itself investigated 55 individual attacks, which killed 152 people, and found that contrary to some claims over the course of the uprising, activists' casualty figures were an underestimate.

It documents eight attacks on four bakeries, which it said killed 35 people and 11 on two hospitals. The Dar al-Shifa Hospital, the main emergency centre in rebel-held Aleppo, and its surroundings were bombed eight times, and a makeshift hospital in Salma, Latakia province, three times.

The regime admitted one of the attacks on Dar al-Shifa, despite hospitals being protected under the Geneva Convention, HRW pointed out.

Many other attacks would count as illegal under the rules of war because of their indiscriminate nature – the Russian missiles and, occasionally, homemade weapons used have no guidance systems, even if rebel units were being targeted. But in any case, the report quoted defecting army officers saying that the regime intended attacks to be inaccurate so as to scare the civilian population and force them to leave.

"Ninety per cent, even 95 per cent of the bombing is random," it quoted one pilot as saying. "What's the purpose? To scare the population, terrorise people, and make them turn their back on the revolution.

"Pilots would dive in, attack, but then go back up without shooting. Then they [the commanders] would force them, ordering them to shoot."